Interlace control for television pictures



Aug. 3, 1954 A.. R. DAVIDSON INTERLACE'CONTROL FOR TELEVISION PICTURES 2Sheets-Sheet l Filed Feb. 27 1950 Srwentor @aqua/1 Gttorneg A. R.DAVIDSON 2,685,610

2 Sheets-Sheet 2 Aug. 3, 1954 INTERLACE CONTROL FOR TELEVISIONPICTURESFiled Feb. 27, 195o Gttorneg Patented Aug. 3, 1954 UNITED STATES PATENTGFFICE INTERLACE CONTROL FOR TELEVISION PICTURES 2 Claims.

In television it is desirable that alternate scanning elds be socontrolled that all horizontal lines in a completed frame are equallyspaced, e. g., in the rst field lines I, 3, 5, and all odd lines arescanned; in the next eld, lines 2, 4, 6, and all even lines are scannedthus completing a picture frame. The spacing between lines I and 2, 2and 3, 3 and 4, etc., are required to be equal by definition ofinterlacing. Difficulty has been experienced in obtaining interlacedpicture patterns in television receivers. This invention is intended toproduce reliable interlacing of the received picture patterns usingl thepresent television transmitting stations. Further objects and advantagesappear in the specication and claims.

In the drawing, Fig. l is a diagrammatic view of a television receivingcircuit equipped with an interlace control, Fig. 2 is a circuit diagramof the interlace control, and Fig. 3 is a circuit diagram of anothertype of vertical sweep oscillator foruse with the Fig. 2 circuitdiagram.

In the standard television broadcast, the horizontal or linesynchronizing pulses are at the rate of 15,750 per second and are spacedso as to provide adjacent picture elds which, when interlaced, wouldform a pattern of 525 lines as a theoretical maximum. The picturesignals indicated at I have an amplitude carrying the pictureinformation in the center of which is a pedestal 2 on which issuperimposed a horizontal synchronizing pulse 3. At the end of eachpicture pattern there is a series of six equalizing pulses 4 followed bya series of six serrated vertical synchronizing pulses 5 and thenanother series of six equalizing pulses 6. The equalizing pulses and theserrated pulses are at the same level as the synchronizing pulses. Theequalizing and serrated pulses are alternately shifted one half thespacing between the horizontal lines on thepicture pattern. Theequalizing and serrated pulses between adjacent picture elds arestaggered so that two picture elds are produced, in each of which thehorizontal lines are interlaced if the information transmitted by thevertical serrated pulses 5 is accurately reproduced in the televisionreceiver. The equalizing pulses 4 and 6 are to provide consistenthorizontal pulse information during the vertical synchronizing interval.The equalizing pulses are also necessary when the conventionalintegrating Circuit is employed for vertical synchronizing to provideconsistent amplitude at the start of the integrating process.

The television picture signals are fed through a picture clipper 1 whichremoves all of thepcture information and passes only the linesynchronizing pulses 3, the equalizing pulses 4 and 6, and the verticalsynchronizing serrated pulses 5,.

The output of the picture clipper is fed to a diierentiating circuitwhich produces in its output positive peaks 3a, 4a, 5a, and 6acorresponding to the beginning of the synchronizing, equalizing, andserrated pulses 3, 4, 5, and 6. The differentiating circuit alsoproduces negative peaks 3b, 4b, 5b, and 6b corresponding to the ends ofthe correspondingly numbered pulses. The negative peaks 3b, 4b, 5b, and6b are ineiective. The positive peaks 3a, 4a, 5a, and 6a fire atriggered square Wave generator 9 which, at each ring, produces in itsoutput positive square waves or control or deletion pulses I0 of longerdurationzthan the synchronizing and equalizing pulses 3, 4, and 6, butof shorter duration than the vertical serrated pulses E. These squarewaves or control or deletion pulses I0 are fed to a normally negativelybiased control tube II in the sense to over'bias the tube. The outputfrom the picture clipper I is fed through a pulse clipper I2 (whichequalizes the height of the pulses) to the control tube II. The overbiasing effect exists for the duration of the square waves or control ordeletion pulse it. In a practical circuit, the duration of the squarewaves or control or deletion pulse IIJ will be of the order of sixmicro-seconds, whereas the duration of the synchronizing and equalizingpulses 3, 4, and B will be of the order of ve micro-seconds. This meansthat the synchronizing and equalizing pulses will have no appreciableeffect upon the control tube II. However, the vertical serrated pulses 5which have a duration of the order of twenty seven micro-seconds willcause the control tube l I to conduct during the last twenty onemicro-seconds of each of the pulses. The output of the control tube IIaccordingly consists of pulses I3 of approximately twenty onemicroseconds duration and corresponding with the vertical serratedpulses 5. The output of the control tube I I is fed through a cathodefollower I4 to a vertical blocking oscillator I5 which generates thesaw-tooth Wave I6 which is fed through a power amplifier Il to thevertical input I8 of a picture tube I9. The synchronizing output of thecathode follower I4 is negative. If a positive synchronizing output isrequired, i4 can be a triode ampliiier.

The pulse clipper I2 also feeds a horizontal synchronizing oscillator 2twhich produces a sawtocth wave 2I which is fed through an amplier 22 tothe horizontal input 23 of the picture tube. The horizontal sweep isconventional.

From one aspect the control for the vertical sweep oscillator isresponsive to the width of the pulses. The narrow synchronizing andequalizing pulses 3, 4, and 6 are of insufficient duration to passcontrol tube Il and control the vertical sweep oscillator I5. The wideror longer duration vertical serrated pulses are effective to fire thevertical sweep oscillator in synchronism with the pulses 5. Of course,the vertical sweep oscillator can be fired only once so only the firstof the pulses 5 is effective.

The interlace control requires no change in television transmission. Thevertical sweep oscillator fires on the rst of the six vertical serratedpulses now transmitted and since the pulses following consecutivepicture patterns are synchronized one half the spacing between thepicture lines, consecutive picture patterns are interlaced. In otherwords, two consecutive iields in eiect form one frame with twice theline density.

Because of the reliability, it is not necessary to have the equalizingpulses 4 and E nor is it necessary to have more than one verticalserrated pulse between each pattern. This would permit the transmissionof more picture information should the television transmission bechanged to eliminate the unnecessary equalizing and vertical serratedpulses.

Another advantage is the elimination of the vertical hold control nowpresent on television receivers.

From one aspect, the interlace control is responsive to pulse width. Thenarrow synchronizing and equalizing pulses are blocked and have noeffect. The iirst of the wide vertical serrated pulses iires thevertical sweep oscillator thereby positively synchronizing theconsecutive picture patterns.

In the circuit diagram, the differentiating circuit il comprises acondenser 8a and a resistance 8b across which the peaks Sc-iib appear.The square wave generator triggered by the peaks 3a, 4a., 5c, comprisesa double triode Sa, 9b, 9b being normally on and lid being normally offand triggered on by the peaks 3a, fic, 5c, (ia. The drop in voltage uponthe firing of 9a is coupled through condenser 9c to the grid 9d of 9b tobias el) off and cause an abrupt rise in voltage at resistance Se. Afteran interval of about 6 microseconds, the grid bias of tube sb rises to apoint at which tube 5b res causing an abrupt drop in voltage atresistance Se which is coupled to the tube Sa to turn it off. Theduration of the abrupt square wave increase in voltage (control ordeletion pulse) lll at the resistance Sie is determined by the circuitconstants. The value of 6 microseconds is chosen for this applicationbecause that is longer than the duration of the synchronizing andequalizing pulses 3, d, 6 and shorter than the vertical serrated pulse5. A 50 micro micro iarad condenser Si across the resistance iig in thefeed back from the plate of the tube Sib to the grid of the tube Ba willincrease the steepness of the lagging slope of the square wave it andeliminate a small initial pulse in the negative direction at thebeginning of the square wave.

The square wave or control or deletion pulse Il! generated at each ofthe pulses 3, 4, 5, 6 is fed to the cathode i la of a control orcomparison tube Ii normally biased to cut oli. Being positive, thisbiases the tube to cut off. The pulses 3, li, 5, 5 are also fed throughthe diode pulse clipper l2 to the grid i lb of the control tube. Thepulses fed through the clipper l2 are of insuiiicient magnitude tooverbalanoe the negative bias due to the square wave control or deletionpulses Ill but are suicient to re the control tube Il in the absence ofa negative bias from the square wave control or deletion pulses ill.Since the square wave control or deletion pulses iii are all of the samewidth, the control tube il is fired only during the latter part of thevertical serrated pulse 5. For the numerical values given, the controltube i l is biased off for 6 micro-seconds which is longer than the 5micro-second pulses 3, Il, and t, and is 2l micro-seconds less than the27 micro-second vertical serrated pulses 5. The control tube Ilaccordingly fires during the last 21 micro-seconds of each of thevertical serrated pulses 5.

The output of the control tube l l, which is synchronized with thevertical serrated pulses 5, is fed to the grid Ma of the cathodefollower i4 having a cathode resistor Mib across which a negativesynchronizing pulse appears and having a plate resistor Mc across whicha corresponding positive synchronizing pulse appears and which may beused with the blocking oscillator shown in Fig. 3. The blockingoscillator controlled by the negative synchronizing pulses from thecathode follower i il comprises a tube lila having its cathode i521directly coupled to the negative synchronizing pulses appearing across'the cathode resistor Illb. The direct coupling has a sharp wave formwhich is important for accurate control oi the blocking oscillator. Sawtooth voltage appears across a condenser i503 which is charged through aresistance H56. As the voltage on the condenser iSd increases, the grid15o becomes more positive so that upon the arrival of the negativesynchronizing pulse at the cathode lh, the condenser iSd is quicklydischarged. Grid current in the tube itc biases the tube oil and thecharging of the condenser ld through the resistor 50. starts again,producing the desired saw tooth wave form. il indicates the conventionalamplifier tube to which the saw tooth wave form is coupled. Once thetube 50. has started to conduct and discharge the condenser ld, the tubeis not susceptible to external control. This makes it important that thetube l5a be iired synchronously with the vertical serrated pulses sothat the saw tooth voltage generated across the condenser 15d will beaccurately synchronized with the transmitted picture patterns. In thepresent circuit the precise synchronization is obtained since the tube55a is always fired in synchronism with the first of the verticalserrated pulses 5. This insures the reproduction in the televisionreceiver of the interlaced picture patterns transmitted.

While the equalizing pulses i and 5 and all but the iirst of thevertical serrated pulses li are unnecessary, the presence of these addedpulses does not interfere with the operation of the present interlacecontrol.

Receivers equipped with the present interlace control can be usedWithout any change of the present television broadcast where consecutiveinterlaced transmitted picture patterns are separated by widetransmitted pulses for synchronizing the receiver so the receiver willreproduce the interlaced transmitted picture patterns.

The output of the described interlace control circuit is negativesynchronizing for direct coupling to the blocking oscillator cathode.This method preserves the synchronizing pulse wave form for precisecontrol. Positive synchronizing for control of other types of controlledsaw tooth oscillators may be secured by using Hi as a triode amplier forphase reversal instead of as a cathode follower as shown. The circuitdiagram accomplishing this is shown in Fig. 3 where the positivesynchronizing pulses appearing at the plate of the tube I4 are coupledto the grid I5c of the blocking oscillator tube I5a. The arrival of thepositive pulses causes the tube I5a to conduct and to short circuit thecondenser I5d across which the saw tooth voltage appears. In otherrespects the operation is the same as in the Fig. 2 circuit.

In some vertical sweep circuits the control tube II and the cathodefollower I4 can be omitted and the vertical sweep circuit fed directlyfrom the square wave generator 9 and the pulse clipper I2. This ispossible Where the sweep circuit design permits synchronizing withoutintermediate amplication. For example, with a multi-vibrator sweepgenerator, the output of the square wave generator 9 will supply apositive pulse I0 to the cathode while the synchronizing input from thepulse clipper I2 is being injected into the grid. Horizontal andequalizing pulses 2, 4, and 6 are over biased by the square wave pulsesI0 even though the grid has a positive pulse apv plied. The verticalserrated pulses are only blanked off for the 6 micro second intervalcorresponding to the duration of the square wave pulses I0 and thebalance of the 21 micro seconds applies the trip pulse.

What I claim as new is:

l. In a television receiver for television signals having interlacedpatterns of horizontal lines separated by vertical synchronizing pulsesof greater Width than the line synchronizing pulses, said verticalsynchronizing pulses being synchronized so consecutive patterns ofhorizontal lines interlace. means responsive to each of the pulses forproducing deletion pulses of less width than the vertical synchronizingpulses but of greater Width than the line synchronizing pulses, avertical sweep circuit, and means for synchronizing the vertical sweepcircuit including comparison means fed in opposed relation by said inputpulses and by said deletion pulses, the input pulses being fed to thecomparison means in the sense to synchronize the vertical sweep circuitand the deletion pulses being fed to theV comparison means in the senseto block synchronizing of the vertical sweep circuit whereby the 6separated by vertical synchronizing pulses of greater width than theline synchronizing pulses, said vertical synchronizing pulses beingsynchronized so consecutive patterns of horizontal lines interlace, avertical sweep circuit oscillator synchronization circuit comprising acomparison control tube for synchronizing the vertical sweep circuitwhen the tube is biased on, means for feeding the vertical synchronizingand line synchronizing pulses to the comparison control tube in thesense to bias the tube on, square wave i generator means for producingcontrol pulses of vertical sweep circuit is synchronized by the exlesswidth than the vertical synchronizing pulses but of greater width thanthe line synchronizing pulses, means for triggering the square Wavegenerator rneans from the leading edge of each of the verticalsynchronizing and line synchronizing pulses and for rendering the squarewave generating means independent of the remainder of said verticalsynchronizing and line synchronizing pulses following the leading edges,means coupling the control pulses produced by the square wave generatormeans to the comparison tube in the sense to bias the tube off, meansfor coupling the vertical synchronizing and line synchronizing pulses tothe comparison tube in the sense to bias the tube on, said controlpulses produced by the square Wave generator means being of sufficientmagnitude to hold the comparison control tube ofi for the duration ofthe control pulses whereby the comparison tube is biased on tosynchronize the vertical sweep oscillator during that portion of thevertical synchronizing pulses which exceeds the duration of the controlpulses produced by the square wave generator means.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,286,450 White et al. June 6, 1942 2,487,682 Wendt Nov. 8,1949 2,491,804 Fleming et al Dec. 20, 1949 2,508,923 Mautner May 23,1950 2,548,219 Jenkins Apr. 10, 1951 2,570,775 De Baun Oct. 9, 1951FOREIGN PATENTS Number Country Date 845,351 France Aug. 21, 1939 OTHERREFERENCES Communications for July, 1938, pages 5-8.

